There may be various reasons why an increased accumulation of fluid occurs in tissue. The most common causes include heart failure, kidney failure or cirrhosis of the liver. In rare cases, oedema can develop following septicaemia. The usual treatment of oedema involves diuretics, i.e. water tablets, which remove excess fluid from the body and which can be administered either orally or intravenously. For some years now, it has also been possible to use ultrafiltration to treat oedema. This involves filtering and removing excess fluid from the blood. This individual method enables a precisely defined amount of fluid to be withdrawn. Access to the blood circulation is usually via a central venous catheter, as in acute dialysis. Current international treatment guidelines recommend that consideration should be given to ultrafiltration therapy in the context of treatment of diuretic-resistant volume overload (e.g. second- or third-line therapy for acute decompensated heart failure). There are, however, to date no clinical data on a combined treatment regimen of diuretics and supportive ultrafiltration. Accordingly, ultrafiltration may be included in clinical guidelines either only with a low level of evidence (e.g. IIb in the ACCF/AHA guidelines) or not at all (ESC guidelines). The main reasons for the limited body of evidence for ultrafiltration are, on the one hand, the invasive nature of the usual procedures (these usually require the insertion of a central venous catheter) and structural barriers in the health system (ultrafiltration is normally offered by nephrologists and not by cardiologists). With an increasing clinical need and limited medical alternatives, particularly in view of the frequently occurring diuretic resistance in heart failure, there is an urgent medical need to fill this gap in evaluation evidence. In the context of the registry, the CHIARA system, a minimally invasive (i.e. via a peripheral venous access) extracorporeal ultrafiltration system, is used for the treatment of decompensated volume overload. The CHIARA system has a CE mark for the intended purpose of ultrafiltration of the blood of patients suffering from heart failure, acute or chronic renal failure or excess body fluid. It is planned to use the medical device in connection with this intended purpose only. In participating hospitals, patients will be treated with this new treatment strategy of minimally invasive ultrafiltration treatment in support of diuretic drug therapy in the acute phase of volume overload. It is possible with the use of ultrafiltration therapy to control volume overload and reduce it on an individual basis, so that diuretics can be given sparingly and, as a consequence, diuretic resistance and a deterioration of renal function due to diuretic uptitration can be avoided.